I made a highly precise (I hope!) emulator of AY-3-8910 and YM2149 sound chips with an audio renderer to wav. It understands YM format (but you may need to unpack a file with LHA tool). No ATARI ST specific effects (I'm from ZX Spectrum land) yet.

it sounds nice, but it can't load compressed YM files (most of them are compressed in LHa, now I see the author told about this in the first post). Just out of curiosity, what is this .fym format? (It seems to be compressed)